1. Field of the Invention
The present invention relates to an electrophotographic photoconductor for use in a laser beam printer, facsimile machine, and digital copier, which photoconductor comprises an electroconductive support, and an undercoat layer and a photoconductive layer successively overlaid on the support in this order. In addition, the present invention relates to a production process of the above-mentioned photoconductor, an electrophotographic image forming method and apparatus using the above-mentioned photoconductor. Further, the present invention also relates to a process cartridge holding therein the above-mentioned photoconductor, which process cartridge is freely attachable to the image forming apparatus and detachable therefrom.
2. Discussion of Background
Basically, an electrophotographic photoconductor comprises an electroconductive support and a photoconductive layer formed thereon comprising a photoconductive material. Further, it is proposed to provide an undercoat layer between the electroconductive support and the photoconductive layer for the following purposes: improving the adhesion of the photoconductive layer to the support, the coating characteristics of the photoconductive layer, the charging characteristics of the photoconductive layer, inhibiting unnecessary charges from injecting from the support into the photoconductive layer, and compensating for any defects on the support.
Methoxymethylated polyamide is conventionally known as a well-balanced material for the undercoat layer as disclosed in Japanese Laid-Open Patent Application 6-202366. However, an N-alkoxymethylated polyamide represented by the above-mentioned methoxymethylated polyamide exhibits high water absorption properties because of the presence of an alkoxyl group in the structure. In the case where a photoconductor comprises an undercoat layer comprising such an N-alkoxymethylated polyamide, the photoconductor properties are largely changed in the repeated use under the circumstances of high temperature and humidity, or low temperature and humidity. Such a drawback results from the increase of water content in the undercoat layer. The above-mentioned photoconductor tends to produce abnormal images with toner deposition on the background and low image density.
In Japanese Laid-Open Patent Application Nos. 2-108064 and 10-268543, and Japanese Patent Nos. 2817421 and 2785282, an undercoat layer for use in the photoconductor consists of a crosslinked methoxymethylated polyamide. However, the photoconductor properties are still dependent on environmental conditions because of insufficient crosslinking in the methoxymethylated polyamide. Further, in this case, the problem of the increase in residual potential is caused when the undercoat layer is thickened. More specifically, the surface of the electrophotographic photoconductor is charged, and exposed to light images according to the electrophotographic process. The light-exposed portion of the photoconductor is made electroconductive, and electric charges can transfer in the photoconductor. Image data can be thus recorded in the form of latent electrostatic images. When the thickness of the undercoat layer exceeds 1.0 xcexcm, the electric charge on the light-exposed portion unfavorably remains on the photoconductor, and the residual potential is increased in the repeated use of the photoconductor. The increase in residual potential, which means a deterioration of the photoconductor, will produce abnormal images.
To solve the above-mentioned problem, it is required that the thickness of the undercoat layer be decreased to 1.0 xcexcm or less when the undercoat layer consists of methoxymethylated polyamide alone. However, a thin undercoat layer cannot effectively make up for the defects on the electroconductive support, such as scratches and surface roughness. To regulate the surface properties of the electroconductive support, the surface treatment steps of cutting and abrasion become necessary, thereby increasing the manufacturing cost of the photoconductor.
In addition, when a photoconductor with a thin undercoat layer is set in an electrophotographic image forming apparatus where a contact type charger is installed, discharge breakdown occurs in the photoconductor, with the result that abnormal images are easily produced. In particular, when the process of reversal development is adapted, the above-mentioned discharge breakdown produces a relatively large black spot image. This is conventionally regarded as a serious problem.
To eliminate the problem caused by the water absorption properties of methoxymethylated polyamide, it is proposed to add a thermosetting resin such as melamine resin to the methoxymethylated polyamide in Japanese Laid-Open Patent Application No. 3-337861 and Japanese Patent No. 2861557. The aforementioned undercoat layer comprising the methoxymethylated polyamide and the melamine resin can solve the problem resulting from the water absorption properties to some extent. However, there still remains the problem that the physical properties of the methoxymethylated polyamide are practically dependent upon temperature and humidity. Therefore, even though the photoconductive layer is provided on such an undercoat layer, the photoconductor properties are still susceptible to temperature and humidity. The result is that abnormal images such as black spots are produced and the image density is lowered when image formation is repeated under the circumstances of high temperature and humidity or low temperature and humidity.
According to Japanese Laid-Open Patent Application No. 5-150535 and Japanese Patent No. 2861557, an undercoat layer for use in the electrophotographic photoconductor comprises (i) a thermosetting resin and (ii) a thermoplastic resin such as a modified polyamide resin which comprises as the main component a copolymer polyamide comprising a modified polyamide 6 or polyamide 6. When such a photoconductor is operated under the circumstances of low temperature and humidity, the residual potential (VL) of a light-exposed portion tends to largely vary and produces abnormal images.
As disclosed in Japanese Laid-Open Patent Application Nos. 61-204642 and 62-280864, it is well known that an inorganic pigment such as titanium oxide is dispersed in the undercoat layer to effectively compensate for the defects on the surface of the electroconductive support and to enhance the light scattering effect of coherent light such as a laser beam and prevent the interference fringes. Such an undercoat layer comprising an inorganic pigment causes no problem in the initial stage. However, when the photoconductor is set in an electrophotographic image forming apparatus and repeatedly used for an extended period of time, defective images such as toner deposition on the background and non-printed white spots in a solid image become conspicuous with time.
To eliminate the defective images produced in the repeated use, there is proposed in Japanese Laid-Open. Patent Application Nos. 63-289554 and 64-031163 an electrophotographic photoconductor comprising an electroconductive support, and a first undercoat layer containing no filler, a second undercoat layer in which an inorganic pigment is dispersed, and a photoconductive layer which are successively overlaid on the electroconductive support. However, such a layered undercoat layer cannot solve the above-mentioned problem. Namely, occurrence of abnormal images cannot be prevented when the photoconductor is used for an extended period of time.
In Japanese Laid-Open Patent Application No. 6-202366 and Japanese Patent No. 2885609, it is proposed to provide an undercoat layer using a coating liquid prepared by dissolving and dispersing non-electroconductive titanium oxide particles and a polyamide resin in a mixed solvent of an alcohol and a particular organic solvent. However, the water absorption properties of the obtained undercoat layer are so high that the photoconductor properties are largely dependent upon environmental conditions. Therefore, black spots will appear and the image density will be lowered in the repeated use of the photoconductor under the circumstances of high temperature and humidity or low temperature and humidity, as mentioned above.
A photoconductor disclosed in Japanese Laid-Open Patent Application 61-036755 comprises a first undercoat layer in which titanium oxide particles coated with a layer comprising Sb2O3 and SnO2 are dispersed and a second undercoat layer consisting of a resin component, the first and second undercoat layers being successively overlaid on the support in this order. However, the overall requirements of the photoconductor, for example, the charging characteristics, sensitivity, and image quality are not satisfied.
In Japanese Laid-Open Patent Application 9-288367, a first undercoat layer comprising a thermosetting resin and an inorganic pigment dispersed therein and a second undercoat layer comprising a polyamide resin are interposed between the electroconductive support and the photoconductive layer. By the provision of the second undercoat layer comprising a polyamide resin, abnormal images can be inhibited from occurring even after the photoconductor is repeatedly used. However, the increase in residual potential of a light-exposed portion is noticeable while in practical use under the circumstances of low temperature and humidity. The potential of the light-exposed portion tends to increase with the increase of the residual potential, and this tendency becomes striking as the photoconductor is caused to deteriorate.
By the way, to carry out the crosslinking of an N-alkoxymethylated polyamide and a melamine resin at a practical temperature, both are dissolved in a solvent to prepare a resin solution, and the resultant solution is made acid and heated. However, when titanium oxide particles are dispersed in the resin solution to which an acid catalyst is added, the obtained dispersion is so unstable that the pot life of the dispersion is short. In such a dispersion, inorganic pigment particles such as titanium oxide particles tend to aggregate to form large number of agglomerates. When an undercoat layer is provided on the electroconductive support using such a dispersion as a coating liquid for undercoat layer, the surface of the obtained undercoat layer cannot be made even because of the presence of the above-mentioned coarse particles of agglomerates. The defects on the electroconductive support cannot be made up for by the provision of the undercoat layer as a matter of course, and the photoconductive layer cannot be uniformly provided on the undercoat layer. As a result, the photoconductor will produce abnormal images such as black spots and images with a low image density because the photoconductor properties are uneven. To solve the above-mentioned problem in the course of the production of the photoconductor, it is required that the coating liquid for undercoat layer be frequently replaced with a new one, whereby the manufacturing cost necessarily increases.
An undercoat layer disclosed in Japanese Laid-Open Patent Application 9-269606 comprises a crosslinked material of methoxymethylated polyamide resin and a melamine resin, and surface-treated titanium oxide particles dispersed in the crosslinked material. In this application, the surface-treated titanium oxide particles are used to improve the dispersion stability of titanium oxide particles in the resin solution. However, the use of such surface-treated titanium oxide particles readily increases the residual potential of the photoconductor after the repeated use. To solve the problem of increase in residual potential, the undercoat layer is required to be extremely thin. In the case where the undercoat layer is extremely thin, the step of regulating the surface properties of the electroconductive support becomes necessary, and abnormal images are easily produced because discharge breakdown occurs in the photoconductor. Further, by the influence of the surface treatment to which titanium oxide particles are subjected, the photoconductor properties are largely dependent upon environmental conditions.
In view of the above-mentioned conventional drawbacks, it is a first object of the present invention to provide an electrophotographic photoconductor with high durability, capable of constantly producing high quality images even though the photoconductor is repeatedly used under the circumstances of high temperature and humidity or low temperature and humidity.
A second object of the present invention is to provide an electrophotographic photoconductor free from the occurrence of discharge breakdown, and the increase in residual potential.
A third object of the present invention is to provide an electrophotographic photoconductor which can be manufactured at low cost.
A fourth object of the present invention is to provide a production process of the above-mentioned electrophotographic photoconductor.
A fifth object of the present invention is to provide an electrophotographic image forming apparatus employing the above-mentioned electrophotographic photoconductor.
A sixth object of the present invention is to provide an electrophotographic image forming method employing the above-mentioned electrophotographic photoconductor.
A seventh object of the present invention is to provide a process cartridge holding therein the above-mentioned electrophotographic photoconductor.
The aforementioned first to third objects of the present invention can be achieved by an electrophotographic photoconductor comprising an electroconductive support, an undercoat layer formed thereon, and a photoconductive layer formed on the undercoat layer, the undercoat layer comprising (a) an inorganic pigment and (b) a binder resin which is selected from the group consisting of a crosslinked N-alkoxymethylated polyamide and a crosslinked material of an N-alkoxymethylated polyamide and a melamine resin.
The undercoat layer may comprise a first undercoat layer and a second undercoat layer which are successively overlaid on the electroconductive support in this order. In this case, the first undercoat layer comprises a thermosetting resin and the above-mentioned inorganic pigment dispersed in the thermosetting resin, and the second undercoat layer comprises the binder resin selected from the group consisting of the crosslinked N-alkoxymethylated polyamide and the crosslinked material of the N-alkoxymethylated polyamide and the melamine resin.
The above-mentioned fourth object of the present invention can be achieved by a method for producing an electrophotographic photoconductor comprising the steps of applying a coating liquid for undercoat layer comprising (a) an inorganic pigment and (b) a binder resin which is selected from the group consisting of an N-alkoxymethylated polyamide and a mixture of an N-alkoxymethylated polyamide and a melamine resin to an electroconductive support to form a coated film thereon, heating the coated film to crosslink the N-alkoxymethylated polyamide or the mixture of N-alkoxymethylated polyamide and melamine resin, thereby providing an undercoat layer on the electroconductive support, and providing a photoconductive layer on the undercoat layer.
In the case where the undercoat layer comprises the first and second undercoat layers, a method for producing the electrophotographic photoconductor comprises the steps of providing on an electroconductive support a first undercoat layer which comprises a thermosetting resin and an inorganic pigment dispersed in the thermosetting resin, applying a coating liquid for second undercoat layer comprising a binder resin which is selected from the group consisting of an N-alkoxymethylated polyamide and a mixture of an N-alkoxymethylated polyamide and a melamine resin to the first undercoat layer to form a coated film thereon, heating the coated film to crosslink the N-alkoxymethylated polyamide or the mixture of N-alkoxymethylated polyamide and melamine resin, thereby providing a second undercoat layer on the first undercoat layer, and providing a photoconductive layer on the second undercoat layer.
The fifth object of the present invention can be achieved by an electrophotographic image forming apparatus comprising the above-mentioned electrophotographic photoconductor, means for charging the electrophotographic photoconductor for forming a latent electrostatic image thereon, and means for developing the latent electrostatic image formed on the electrophotographic photoconductor to a visible image.
The sixth object of the present invention can be achieved by an electrophotographic image forming process comprising the steps of forming a latent electrostatic image on the surface of the above-mentioned electrophotographic photoconductor, and developing the latent electrostatic image to a visible image by reversal development.
The seventh object of the present invention can be achieved by a process cartridge which can be freely attachable to an electrophotographic image forming apparatus and detachable therefrom, the process cartridge comprising the above-mentioned electrophotographic photoconductor, and at least one of a charging means for charging the surface of the photoconductor, a light exposure means for exposing the photoconductor to a light image to form a latent electrostatic image on the photoconductor, a developing means for developing the latent electrostatic image to a visible image, or an image transfer means for transferring the visible image formed on the photoconductor to an image receiving member.